1. Field of the Invention
The present invention relates to a separate function type of electrophotographic photosensitive member having a charge generation layer and a charge transport layer formed by dip coating.
2. Description of the Prior Art
It has been widely practiced up to now that a resin layer or a photosensitive layer is formed on a substrate by coating to produce an electrophotographic photosensitive member. Among various coating methods, the dip coating method that comprises dipping a substrate in a coating solution and taking up the substrate is specially favorable since the substrate in arbitrary form can be neatly coated thereby.
In dip coating, the coating thickness, when the coating material is given, depends on the concentration thereof and on the take-up speed. It is known that the coating thickness increases with the concentration and with the take-up speed.
However, when the take-up speed is high, the coating material on the substrate sags before setting by drying, thus forming an uneven coating which becomes thinner upward, i.e. becomes thicker downward. In particular, when the concentration of active materials in the coating solution, the sagging tends to occur even if the viscosity of the coating solution is raised, because of a large amount of solvents in the coating solution.
This tendency is remarkable specially in the coating to form charge transport layers of the separate function type electrophotographic photosensitive members having charge generation and charge transport layers. Generally the charge transport layer is formed by applying an electron donative material or electron attractive material together with a film-formable resin dissolved in a solvent, where the solvent needs to be used in a large proportion because electron donative or attractive materials commonly have low solubilities in solvents. Consequently the coating solution for the charge transport layer is dilute, and for the purpose of thickening the charge transport layer to a certain extent, the viscosity of the coating solution is raised. When such a dilute coating solution is applied on a substrate by dip coating, a large portion of the coat, before setting, descends the substrate during the take-up thereof, since the drying of the coat is slow because of the high concentration of solvents. This phenomenon generates an uneven coating thickness as shown in FIG. 1, where the distance from the upper end of the coat is plotted as abscissa and the thickness of the formed charge transport layer as ordinate. Thus the coating thickness decreases upward.
Such uneven thickness of the charge transport layer causes irregularity in the acceptance potential, which is a characteristic of the electrophotographic photosensitive member. That is, the potentials on thinner portions of the layer are lower than those on the other portions. In addition, the thinner portions show relatively quick response (high sensitivity) to light exposure since the lower potentials of the thinner portions readily decrease on light exposure.
In such a case, while the low acceptance potential can be offset to maintain the reproduced image density constant, by the method of applying a bias voltage for toner development, the quicker response cannot be compensated. Accordingly, even though the black portions of the reproduced images can be kept at a constant image density, the reproduced images of plotting paper, photographs, catalogues, posters, pencil-written letters, etc. having half-tone show low image densities on the thinner portions of the coat, forming obscure images.